Grinding is a well known machining technique which is widely used with many materials. However, grinding of titanium has long been a difficult task which is rarely accomplished with the necessary efficiency and the desired ground surface properties.
Titanium is strong but not particularly hard, it is tough, it conducts heat poorly and it is quite chemically reactive. This combination of properties makes grinding difficult. While harder, less tough materials easily form discrete chips, the combination of high toughness and chemical reactivity in titanium leads to "loading" of the grinding wheel with the removed titanium. When the wheel becomes fully loaded or contaminated with titanium, the grinding process essentially ceases and what continues is metal to metal friction with smearing of the workpiece and possible titanium combustion. The smearing process is exaggerated because the low thermal conductivity of titanium causes the grinding wheel/titanium interaction point to reach a high temperature where the titanium becomes relatively soft and even more reactive.
To counteract these problems it has generally been taught in the art to use slow grinding wheel speeds and/or low metal removal rates. This minimizes the buildup of titanium on the grinding surface however, it leads to greatly reduced efficiencies.
Various technical and journal articles suggest that it is fairly conventional in the art to use grinding wheel surface speeds ranging from about 18 to about 92 meters per second (1100-5500 surface meters per minute) in combination with cut depths on the order of 0.025 mm. The journal articles deal mainly with vitrified wheels which have low thermal conductivities and are therefor prone to heat buildup.
The teachings in the technical journals lead to painfully slow removal rates.
Another important aspect of grinding metals is the condition of the resultant ground surface. Mechanical machining processes invariably produce a surface having residual stresses. Such stresses can be compressive or tensile. Tensile stresses are highly deleterious to fatigue life while compressive stresses can improve the fatigue life over that which would be obtained if the surface was stress free.
Surface microstructure is important since the presence of an alpha phase surface layer (alpha case) or a deformed surface microstructure is detrimental to the mechanical properties of the ground article. Surface microstructure problems can result from overheating during grinding.